1. Field of the Invention
The present invention relates to a ratchet assembly device for fitting a back cover and/or a bezel onto a watch case middle, in which at least one elongated elastic assembly element is combined with at least one elongated receptacle made in an assembly side face of one of the members to be assembled and forming in said assembly side face an opening through which a side portion of the surface of said elongated element projects for engagement by elastic deformation with a ratchet fastening element, fixedly connected to the other member to be assembled, upon the relative translatory displacement of these two members to fit them together, at least one of the faces of said ratchet fastening element being configured to hold said elastic assembly means under elastic strain and being orientated to resolve the force exerted upon it into a component parallel to said translatory displacement and the direction of which tends to hold said assembled members in place.
2. Description of Related Art
There are a large number of watch cases in existence, especially watertight watch cases in which the members to be fitted onto the case middle, the back cover and/or the bezel, are ratchet fastened. This fastening method is used, in particular, for watertight, so-called shape watches, that is to say watches of polygonal shapes, on which assembly cannot be realized by screwing a thread made on the back cover into a thread made on the case middle.
Such a fastening method is described, in particular, in CH 556 567, relating to a shape case in which a catch is made on the inner side face of the case middle by two faces inclined in opposite directions, an inwardly inclined surface followed by the other surface of opposite inclination, such that a constriction is made at the junction between these two surfaces. The back cover has a rim extending toward the interior of the case and the outer surface of the upper end of this rim forms the assembly surface. The distance between two opposing assembly surfaces of the back cover must be slightly greater than the corresponding dimension of the constriction made at the junction of the inclined surfaces of the inner side face of the case middle. When the back cover is introduced into the case middle, the rim is deformed, and at least partially resumes its original position once it has passed beyond the constriction, thus detaining the back cover on the case middle. A seal is accommodated in the outer side face of the rim of the back cover, beneath the assembly surface, such that this seal is compressed against the assembly surface of the outwardly inclined case middle.
The drawback of such an assembly system derives from the fact that the back cover is not necessarily made of a material chosen for its elastic properties, such that the rim of the back cover, whose elastic deformation serves to allow the ratchet fastening, does not confer good elastic deformation properties. Consequently, since the elastic limit may not be exceeded, the margin of deformation for the coupling is necessarily small. Considering, on the one hand, the small deformation permitted for the coupling and, on the other hand, the production tolerances, it is then no longer possible to fit any back cover to any case middle, which may dictate that the back covers and the case middles are classified according to the actual dimensions of the distances between the respective opposing assembly surfaces of the members to be assembled, or to their diameters in the case of round cases. Such a task clearly constitutes a constraint which is not easy to manage. In addition, depending on the materials used, the coupling properties of such an assembly method may vary, especially according to the number of times that the back cover of the case is opened and closed.
In this type of assembly, moreover, involving watertight watches, the compression of the seal generally exerts a force opposite to the closing force, as in FR 1 387 021, which can adversely affect both the correct closure of the back cover and the watertightness when this compression results from a ratchet fastening rather than a screw fastening.
The use of an elastic closing element independent of the members to be assembled has been proposed, as in CH 423 637 or in CH 447 046. The drawback of the elastic elements proposed in these documents lies essentially in their bulk. In CH 512 769, it has also been proposed to use pins made of compressible materials, surrounded by ring segments which cooperate with mill-cuts made in the angles of a square or rectangular case. Such a device can only be used on shape cases and the elasticity given by an elastomer-type compressible material is likely to alter considerably as the material ages.
Finally, in U.S. Pat. No. 2,858,663, a ratchet closing device for a watch case back cover has been proposed, comprising a polygonal split ring accommodated in a groove made in an inner side face of the case middle. The tops of the polygonal sides of this ring rest in the bottom of the groove, whereas the central portions of these polygonal sides protrude from this groove and extend inside the opening in the case middle intended to receive the back cover of the case. The latter has a first frustoconical surface, the top of which is situated above the case middle, followed by a second frustoconical surface, the top of which is situated beneath the case middle. When the back cover is ratchet fitted onto the case middle, the first frustoconical surface serves to return the central portions of the polygonal ring into the groove in the case middle, whereas the second frustoconical surface serves to press the back cover axially against a seal accommodated in the bottom of the opening in the case middle intended to receive the back cover of the case.
In such a device, the axial force which presses the back cover against the seal results from the mere bending of the sides of the polygonal ring. On the other hand, in the absence of any further clarification, it is incomprehensible how this fastening method by bending of polygonal segments in a circle could be used in the case of a square or rectangular case.
A bayonet fastening device, in which two fastening lugs fixedly connected to the back cover of the case engage with two portions of an elastic split ring projecting inside the case middle, is described in U.S. Pat. No. 2,737,010. In the closing position, the frustoconical surface of the back cover engaging with the split ring can produce an axial pressure to compress a seal.
Apart from the fact that only the bending of the elastic closing elements is used, it can be seen that the axial force component resulting from this bending generally serves, moreover, to compress a seal, hence to reduce the axial force used to fasten the back cover onto the case middle. Pending proof to the contrary, moreover, the last two aforementioned solutions are not apparently applicable to square or rectangular cases.